1. Field of the Invention
This invention relates to a measuring device and relates in particular to a device for measuring deployment and operating forces on a well logging instrument.
2. Description of the Related Art
In the deployment of well logging instruments and devices in wells, it is desired to remotely monitor and quantify the forces applied to the instrument string by the various deployment means such as wire line/armored cable with or without assistance of well tractor, caterpillar, worm, crawler, mule, or other push/pull devices; pipe conveyed; or coiled tubing conveyed. A downhole force gage is used for sensing and monitoring the forces applied to the instrument string.
Existing downhole force gages, also called cable head tension sensors, typically employ strain gage sensors to monitor the mechanical strains induced by deployment forces. The strain gages are mounted on a high strength body which is housed in a sealed internal cavity of the gage assembly. The strain gages are attached and bonded with adhesive or other techniques to the strain gage body and configured electrically as a balanced bridge circuit. Mechanical strain proportional to the applied tension or compression load is induced into the strain gage body. With the bridge circuit powered by a constant, regulated d.c. voltage (typically 10 volts), the strain gage bridge outputs a signal (typically in millivolts) proportional to the applied loads.
When submerged in a fluid filled borehole, hydrostatic pressure impinges on the downhole instrument string and force gage assembly, and produces an external differential pressure force which acts upon the force gage assembly. These hydrostatic pressure forces induce undesired proportional offsets in the strain gage output, so a pressure equalizing system is utilized to eliminate the effects of hydrostatic pressure.
A typical force gage assembly is configured with a suitable floating piston (or an elastic bellows), and the internal cavity of the assembly is filled with a suitable hydraulic fluid. The floating piston (or elastic bellows) moves to accommodate any changes in the volume of the hydraulic fluid in the internal cavity due to changes in hydrostatic pressure or due to changes in temperature. By this means the internal cavity of the force gage assembly is thus pressure-equalized to external hydrostatic pressure, and also by this means the internal cavity, together with the strain gage bridge circuits and wiring, are protected from direct contact with the borehole fluids.
However, the typical configuration is complex, has relatively high cost of manufacture, has relatively high cost of maintenance, and requires hydraulic fluid filling of the force gage assembly. The strain gages are in contact with hydraulic fluid which can be a path of electrical leakage, and over time the hydraulic fluid can attack and degrade the strain gage adhesive bonds. The strain gages also are exposed to hydrostatic pressure which induces some inaccuracy in the output signal. Therefore, there is a demonstrated need for a force gage that eliminates the effects of downhole pressure while maintaining the sensing elements in a gas filled chamber.
The present invention addresses the above-noted and other deficiencies in the prior art and provides a downhole force gage for measuring both compression and tension forces on a well logging instrument string.
This invention provides more accurate load measurement by isolating the strain sensing elements from all effects of downhole pressure. The sensing elements are disposed on a load rod and are located in an atmospheric pressure housing. The strain sensing load rod is pressure balanced by suitable selection of multiple seal diameters such that the external pressure loads on the load rod are canceled out essentially eliminating the effects of downhole pressure on the load measurement.
In one aspect of the invention, strain gages are adhesively bonded to the sensing member to form a conventional bridge circuit.
In another embodiment, strain gages are vacuum deposited on the sensing member.
Examples of the more important features of the invention thus have been summarized rather broadly in order that the detailed description thereof that follows maybe better understood, and in order that the contributions to the art may be appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended hereto.